Did she actually fall from a tree, or were her bone fractures the byproduct of fossilisation?

UT Austin professors John Kappelman and Richard Ketcham examine casts of Lucy while scanning the original fossil (background).

Marsha Miller

One of the most famous fossils in human evolutionary history is at the center of a new scientific debate. The fossilized skeleton dubbed "Lucy" was part of an extinct species called Australopithecus afarensis, an early relative of Homo sapiens who was among the first hominins to walk upright. She died 3.18 million years ago, and her remains were discovered in the early 1970s in Ethiopia. Her skeleton is complete enough to give us a good picture of her anatomy, which is part of what led to the current controversy. A study published in Nature this week suggests that a careful analysis of her bones reveals how she died—by falling to her death from a very tall tree. But other scientists say the evidence is thin at best.

University of Texas-Austin anthropologist John Kappelman and his team did a complete X-ray CT scan on Lucy's bones, allowing them to create high-resolution 3-D renders as well as 3-D printouts of her skeleton. By comparing the way her bones had fragmented with contemporary X-rays from people who fell, they came to the conclusion that the fragmentation of her leg bone was "green," that is, it took place right before she died.

Kappelman and his colleagues write, "Although the fractures in Lucy’s humeri provide evidence that she was conscious when she stretched out her arms in an attempt to break her fall, the severity of the numerous compressive fractures and presumed organ damage suggest that death followed swiftly." It appears that the joint in her leg suffered from extreme compression of the type you'd expect in somebody who fell on their feet from a great height, out of a local tree where nests might be as many as 23 meters off the ground. (They estimated this height based on the typical heights of chimpanzee nests today.)

Here you can see an illustration of what Lucy's fall would have been like. "We hypothesize that Lucy fell from a tall tree, landing feet-first and twisting to the right, with arrows indicating the sequence and types of fractures," write the authors in Nature.

Nature

University of Wisconsin-Madison anthropologist John Hawks posted an essay about why Kappelman's analysis is problematic, especially given that he and his colleagues didn't compare the fractures in her bones to other fossils from the same era. Hawks points out that there is a much simpler explanation for Lucy's "injuries" than a severely traumatic fall: "becoming a fossil." The process of fossilization often fragments bones in exactly the way that Lucy's bones are broken, and animals who were fossilized at the same time as Lucy have similar fractures. So Hawks isn't discounting the idea that Lucy died of a fall, but he believes that we need more evidence before confirming it.

If we can confirm that Lucy fell, this discovery provides further evidence for the hypothesis that Lucy's species of Australopiths was accustomed to living in trees as well as walking on the ground. It's likely that Lucy's bipedalism hindered her ability to climb around in trees as easily as other hominins at the time. And that probably meant her species was prone to falling out of trees. Essentially Lucy was at an awkward in-between phase of hominin evolution when our ancient relatives stood on two legs but still lived and slept in trees.

Perhaps what's most exciting about all of this is not so much the mystery of Lucy's death, but how it's being solved. It's entirely due to open science, which is what enabled Kappelman and his colleagues to get such good 3-D printouts of Lucy's skeleton in the first place. Kappelman said in a release that "this is the first time 3-D files have been released for any Ethiopian fossil hominin, and the Ethiopian officials are to be commended. Lucy is leading the charge for the open sharing of digital data." Any researcher can gain access to these files and continue sleuthing. Indeed, the puzzle of Lucy's death may be the first ever solved by what amounts to scientific crowdsourcing.

Annalee Newitz
Annalee Newitz is the Tech Culture Editor at Ars Technica. She is also the author of Scatter, Adapt, and Remember: How Humans Will Survive a Mass Extinction, and her first novel will be published in 2017. Emailannalee.newitz@arstechnica.com//Twitter@annaleen